Certain measurement techniques, like for example Particle Image Velocimetry (PIV), rely on recording two pictures or frames of short exposure time in fast succession. The objects to be imaged—in case of PIV these are particles following a flow of interest—are illuminated in fast succession by one or two pulsed light sources, like for example pulsed lasers. Each of the two light pulses defines the effective exposure time of one of the two frames.
It is known to use so-called “double shutter” cameras for double frame recordings. A double shutter camera records the first frame at a short but the second frame at a long overall exposure time. This means that the image sensors of a double shutter camera are not only registering ambient light during the second pulse of light but for a much longer interval of time. Thus, using a double shutter camera at high ambient light intensities is difficult or even impossible. High ambient light intensities are, for example, present with so-called in-flight measurement techniques implemented at daylight.
Known double shutter cameras have an image sensor comprising light sensors and shift registers including two register places per light sensor. This image sensor is commonly used in that, after each exposure time, the electric signals from the light sensors are transferred to the respective first register places, and that the electric signals are then shifted forward register place by register place to read them out of the shift register. Consequently, electric signals from a first exposure time have to be completely read out of the shift register before electric signals from a second exposure time can be transferred from the light sensors to the respective first register places. Thus, the second exposure time may directly follow the transfer of the electric signals from the light sensors into the shift register after the first exposure time; the second exposure time may, however, not be terminated before the electric signals from the first exposure time have been completely read out of the shift register.
In so far as disturbing ambient light occurs before the light pulses, it can be suppressed by clearing the individual light sensors prior to the desired beginning of the first or second exposure time. Thus, the beginning of the first and second exposure time may be freely selected. The end of the first exposure time may also be freely selected by transferring the electric signals from the individual light sensors to the respective first register places. The end of the second exposure time, at which the electric signals from the individual light sensors are transferred to the respective first register places, may only take place after the electric signals from the first exposure time have been completely read out of the shift register so that the shift register is ready for accepting the signals from the second exposure time.
Generally, ambient light occurring during the second exposure time after the second light pulse may be suppressed by additional fast shutters which are arranged in front of the image sensor of the camera and thus upstream of an electronic shutter of the camera. Mechanical shutters, however, are unsuitable for this purpose due to their inertia and size. Electronic shutters, like for example those of the Micro Channel Plate (MCP) or Photo Multiplier type, are fast enough but reduce the image quality and require high constructional efforts.
The terms “high speed imaging device” and “time-of-flight sensor” designate special image sensors for recording two frames at short exposure times in fast succession, and U.S. Pat. No. 6,972,795 B1 discloses such a special image sensor. Special image sensors designed for a particular purpose, however, are no standard components which are sold in high numbers and which are thus available at low cost.
Particularly, the special image sensor disclosed in U.S. Pat. No. 6,972,795 B1 comprises one register place adjacent to each light sensor and a separate signal transfer line directly connected to each register place. This construction of the light sensor, however, results in a reduced fill factor and in a reduced spatial resolution, which both reduce the evaluation quality when using this image sensor in PIV, for example.
For the purpose of recording two frames at a short exposure time in fast succession with a common image sensor, it is also known to subdivide the image sensor with respect to its lines of light sensors, and to record the first frame with the lines of even line numbers and the second frame with the lines of odd line numbers. Here, however, only half of the available light intensity is used for each frame, and the frames recorded by this method called interlacing are not congruent but display an offset of the line spacing.
Nasibov, Humbat; Kholmatov, Alisher; Akselli, Basak et al.: Performance Analysis of the CCD Pixel Binning Option in Particle-Image Velocimetry Measurements. IEEE/ASME Transactions on Mechatronics, Vol. 15, No. 4, August 2010, pp. 527-540 and Akselli, B.; Kholmatov, A.; Nasibov, K.: The use of CCD pixel binning in PIV measurements. International Symposium on Optomechatronic Technologies, Istanbul, September 21 to Sep. 23, 2009. ISOT 2009. Conference Proceedings, pp. 223-228 disclose another method of using a common image sensor for recording pictures in fast succession. Here, charges from a plurality of horizontally or vertically neighboring pixels are combined or added. This step called “binning” is executed after the respective exposure time by operating the vertical and horizontal registers of a CCD chip used as the image sensor in a special way prior to digitizing the charges. There is, however, no description of how this special way of operating the vertical and horizontal registers has to take place. By “binning”, the time needed for reading all the charges corresponding to one frame out of the CCD sensor is accelerated such that its shift registers are earlier ready for accepting new charges corresponding to a next frame.
Truesense imaging, Inc. (Ed.): KAI-0330 Image Sensor. Device Performance Specification. Revision 1.0 PS-0023, Jul. 13, 2012. PS-0023, Revision 1.0. Rochester, N.Y.: Truesense Imaging, Inc., 2012 disclose an image sensor designated as KAI-0330. This image sensor comprises shift registers with two register places per each light sensor of each row of light sensors. This known image sensor allows for binning the charges of two adjacent lines of light sensors during a transfer of the charges from its vertical shift registers to its horizontal shift register.
Theuwissen, Albert J. P.: Solid-State Imaging with Charge-Coupled Devices. Reprinted with corrections 1996. ISBN 0-7923-3456-6. Dordrecht, Boston, London: Kluwer Academic Publishers, 1996. Chapter 6: Solid-State Imaging for Television Applications, pp. 157-191 describes different ways of using an image sensor comprising one shift register for each row of its light sensors. The shift register is only able to separately hold one charge per two light sensors in a row. Thus, at the end of each exposure time, either only the charges from the light sensors in every second line or the combined charges of two neighboring light sensors can be transferred into the shift registers. In the latter case called “field integration”, it is avoided that only the charges accumulated in half of the light sensors during each exposure time are evaluated and that, thus, each frame has holes between the presently read-out light sensors. In field integration the pairs of neighboring light sensors whose charges are combined change within the vertically oriented rows of light sensors upwards and downwards. In both ways of using a common light sensor described here, two successively recorded frames display an offset, and the entire first frame has to be read out of the image sensor prior to terminating the second exposure time. Theuwissen also describes how to use a so-called progressive scanning image sensor having shift registers simultaneously accepting charges from all light sensors and keeping them separately. Here too, the shift registers have to be completely read out before they can accept charges from the light sensors at the end of a next exposure time.
US 2007/0002165 A1 discloses a method of capturing a sequence of images in close succession with a CCD image sensor. In this method, only the charges of every second light sensor in a row are transferred into the associated shift register after a first exposure time. The other charges are disposed when clearing the light sensors prior to the second exposure time. At the end of the second exposure time the charges collected by the other light sensors whose charge have not been used at the end of the first exposure time are transferred into the shift registers onto those register places which are still empty. Then the charges from the first and the second exposure time are read out of the shift registers. During that time, a further frame may be recorded. The charges from this further frame, however, may only be transferred into the shift register after it has been completely read out from the charges from the first and second exposure times. The first and second frames recorded according to this method display an offset of one line spacing. Further, both frames display holes as their charges are only stemming from every second line of light sensors.
There still remains a need of a method of using a common image sensor for recording a plurality of frames at a short exposure time in fast succession without offset between successive frames and holes in individual frames, and of a camera comprising a common image sensor and implementing such a method.